Process for Producing Capacitors

ABSTRACT

The invention relates to a process for producing capacitors based on niobium suboxide, and having an insulator layer of niobium pentoxide. Also described is a powder mixture suitable for production of capacitors. Pressed bodies produced from the powder mixture, and capacitors having specific properties are also disclosed.

CROSS REFERENCE TO RELATED PATENT APPLICATION

The present patent application claims the right of priority under 35U.S.C. § 119 (a)-(d) of German Patent Application No. 103 33 155.7,filed Jul. 22, 2003.

1. Field of the Invention

The invention relates to a process for producing capacitors based onniobium suboxide with an insulator layer of niobium pentoxide, to apowder mixture suitable for production of capacitors, to pressed bodiesproduced from the powder mixture and to capacitors having specificproperties.

2. Background of the Invention

In the context of the present invention, the term niobium suboxide is tobe understood as meaning compounds of the formula NbO_(z) where z<2.2and preferably 0.5<z<2.2.

Solid electrolyte capacitors with a very large active capacitor surfacearea and therefore a small overall size that is suitable for mobilecommunications electronics used are predominantly those with a niobiumor tantalum pentoxide barrier layer applied to a correspondingconductive support, utilizing the stability of these compounds (“valvemetal”), the relatively high dielectric constant and the fact that theinsulating pentoxide layer can be produced electrochemically with a veryuniform layer thickness. Metallic or conductive lower oxidic (suboxide)precursors of the corresponding pentoxides are used as carriers. Thesupport, which simultaneously forms a capacitor electrode (anode),comprises a highly porous, sponge-like structure which is produced bysintering extremely fine particulate primary structures or secondarystructures which are already in sponge-like form. The surface of thesupport structure is electrolytically oxidized (“formed”) to give thepentoxide, with the thickness of the pentoxide layer being determined bythe maximum voltage used for the electrolytic oxidation (“formingvoltage”). The counterelectrode is produced by impregnating thesponge-like structure with manganese nitrate, which is thermallyconverted into manganese dioxide, or with a liquid precursor of apolymer electrolyte and polymerization. The electrical contacts to theelectrodes are formed on one side by a tantalum or niobium wire sinteredin during production of the support structure and the metallic capacitorcasing, which is insulated from the wire.

The capacitance C of a capacitor is calculated using the followingformula:

C=(F·ε)/(d·V_(F))

where F is the capacitor surface area, s is the dielectric constant, dis the thickness of the insulator layer per V of forming voltage and VFis the forming voltage. Since the dielectric constant ε is 27.6 fortantalum pentoxide and 41 for niobium pentoxide, but the growth in layerthickness per volt of forming voltage d is 16.6 and 25 Å/V respectively,both pentoxides have virtually the same quotient ε/d=1.64 and 1.69respectively. Capacitors based on both pentoxides, with the samegeometry of the anode structures, therefore have the same capacitance.Differences encountered in details of specific weight-relatedcapacitances are trivial, resulting from the different densities of Nb,NbO_(x) and Ta. Therefore, anode structures made from Nb and NbO_(x)have the advantage of saving weight when used, for example, in mobiletelephones, which strive for every single gram of weight saved. For costreasons, NbO_(x) (Niobium suboxide) is more favourable than Nb, sincepart of the volume of the anode structure is formed by oxygen.

One drawback of niobium suboxide as support body for capacitor barrierlayers is that a sufficient compressive strength of the sintered anodebody and a sufficient wire tensile strength are only achieved bysintering the pressed bodies at a relatively high sintering temperature(in the region of 1450° C. compared to 1150° C. in the case of Nbmetal). The high sintering temperature leads firstly, as a result ofincreased surface diffusion, to a decrease in the surface area of thepressed body during transition to the sintered body, and therefore to alower capacitance, and secondly requires increased levels of energy andincreased loading being applied to the materials of the crucibles andsintering furnaces.

The reason is that niobium suboxide, by comparison with niobium metalwith metallic ductility, already has considerable covalent bond levels,which produce in relative terms a ceramic brittleness.

Furthermore, the compressive strength of the anode bodies prior tosintering leaves something to be desired, since the porous powderagglomerates do not stably “mesh together” during pressing, but ratherhave an increased tendency to disintegrate or abrade, with the resultthat not only is the formation of stable sintered bridges impeded, butalso agglomerates in a more finely particulate form, even down toisolated primary particles, are formed, causing an adverse change in thepore structure of the sintered anode body. Furthermore, there isincreased wear to the press tools be comparison with metal powders. Byno means least, niobium oxide powders also have worse flow propertiesthan metal powders, making it more difficult to meter the powders intothe press tools.

According to WO 01/71738 A2, therefore, it is attempted to relieve themagnitude of these drawbacks by on the one hand adding lubricants andbinders during pressing of the powders, which are intended to compensatefor the drawback of insufficient compressive strength of the pressedbodies, and on the other hand by using more finely particulateagglomerates of primary particles, which are less likely to fracture,but this is to the detriment of the pore structure.

SUMMARY OF THE INVENTION

It is an object of the present invention to avoid the drawbacks incapacitor production which are caused by the brittleness of niobiumsuboxide.

Accordingly, it is an object of the invention to improve the flowproperties of the powders during production of niobium suboxide anodes.

Furthermore, it is an object of the invention to provide a powder forproducing capacitor. anodes based on niobium suboxides which can bepressed to form pressed bodies with a high compressive strength.

Another object of the invention is to provide a powder for theproduction of capacitor anodes based on niobium suboxides which can besintered at a relatively low sintering temperature.

Furthermore, it is an object of the invention to provide anodes forcapacitors based on niobium suboxide with an increased compressivestrength of the sintered body.

Not least, it is a further object of the invention to reduce the numberof steps required to produce capacitors based on niobium suboxide andthereby on the one hand to contribute to reducing costs and on the otherhand to reduce the risk of contamination with impurities which have anadverse effect on the capacitor properties, in particular with regard tothe residual current.

It has been discovered that these and further objects can be achieved byvirtue of powder mixtures of niobium suboxide and niobium metal and/ortantalum metal being used as starting material for the production of thepressed and sintered bodies.

Accordingly, the subject matter of the invention is a process forproducing capacitor anodes based on niobium suboxide by pressingsuitable starting materials in powder form to form powder preforms andsintering the powder preforms to give porous anode bodies, which ischaracterized in that the pulverulent starting material used is a powdermixture of niobium suboxide powder and valve metal powder.

DETAILED DESCRIPTION OF THE INVENTION

Niobium and/or tantalum metal powder, preferably niobium metal powder,can be used as valve metal powder.

Both the niobium suboxide powders and the niobium metal powders are usedin the form of the agglomerates of primary particles which are customaryfor capacitor production. The primary particles have the standardminimum linear dimensions of 0.4 to 2 μm. The agglomerates have particlesizes with a Mastersizer D₅₀ value (ASTM B 822) of 30 to 200 μm,preferably 120 to 180 μm.

The niobium suboxide powder used is preferably a powder of formulaNbO_(x) where x<2.1, particularly preferably where 0.7<x<2.

The oxygen content of the starting oxide (“x” in the above formula) andthe relative quantities of niobium suboxide and niobium metal areselected as a function of the desired procedure and the desired product(capacitor). It is desirable for niobium oxide that is present in thesupport structure of the capacitor (the anode) to have the compositionNbO_(y) where 0.7<y<1.3, preferably 0.9<y<1.15 . particularly preferably1.0<y<1.05 . The anode may consist entirely of NbO_(y). However, theanode may also have geometric regions which consist of niobium metal orvery slightly oxidized niobium metal.

According to a first embodiment of the invention, a niobium suboxidepowder of the preferred composition NbO_(y), with y as defined above, ismixed intensively with a niobium metal powder, and the mixture is thenintroduced into a press mould around a niobium or tantalum contact wirein a manner known per se, pressed to a green density of 2.3 to 3.7 g/cm³and then sintered under high vacuum to form anodes.

The pressed bodies have a high sintering activity, on the one hand onaccount of the presence of niobium metal, which has a higher sinteringactivity, but on the other hand also on account of oxygen exchange atthe contact locations between metal and oxide (“reaction sintering”).According to the invention, therefore, sintering temperatures of from1150 to 1300° C. are sufficient, i.e. the process according to theinvention allows sintering temperatures which are lower by 150 to 250°C. to be used.

Niobium metal powder and niobium suboxide powder can be used in anydesired quantitative ratio relative to one another, although the effectof the invention disappears at extreme quantitative ratios. Aquantitative ratio of from 0.1 to 2 (by weight) is preferred, with from0.1 to 0.8 being particularly preferred and 0.2 to 0.4 being even morepreferred.

The particle size distribution may (given an approximately equal primaryparticle size) be selected to be similar. In this case, metal powder andsuboxide powder are preferably used in approximately equal quantitativeratios, for example approximately with a ratio in the range from 40:60to 60:40.

It is preferably for the agglomerate particle size of the metalparticles to be smaller than that of the suboxide particles. By way ofexample, the D50 value (according to Mastersizer, ASTM B 822, wettingagent Daxad 11) of the metal particles may be between 20 and 40 μm,whereas the D50 value of the suboxide particles may be between 130 and190 μm. In this case, it is preferable for the metal powder to be usedin subordinate quantities by comparison with the suboxide powder,preferably with a ratio in the range from 9:91 to 20:80.

According to a second embodiment of the invention, the suboxide andmetal powder agglomerate are intensively mixed, if appropriate withmilling, preferably together, and are then agglomerated, so thatagglomerate powders which include both oxidic and metallic regions areformed. The agglomeration preferably takes place at temperatures between850 and 1200° C. in an inert, preferably argon, atmosphere, so thatthere is no oxygen exchange between the oxidic and metallic particlesapart from at the direct locations of contact through solid-statediffusion. Preferred and particularly preferred suboxide powders areselected according to the same rules as in the first embodiment of theinvention. A starting suboxide NbO_(x) where x is slightly above 1 isparticularly preferred.

After the milling, preferably together, the powders have a preferredparticle size distribution which is characterized by a D50 value of from20 to 50 μm. The D90 value should preferably be less than 90 μm. Afterthe agglomeration, which may if appropriate be repeated a number oftimes, the powders should have a preferred particle size distributionwhich is characterized by a D10 value of from 50 to 90 μm, a D50 valueof from 150 to 190 μm and a D90 value of from 250 to 290 μm.

It has been found that in particular if the agglomeration treatment isrepeated at least twice, with a milling operation in between, thedesired formation of sintering bridges between suboxide and metal powderparticles is promoted, since the intermediate milling preferentiallybreaks up oxide-oxide sintered bridges which have just formed during thepreceding agglomeration step.

The relative quantitative ratios of suboxide and metal particles maypreferably be selected on the basis of same criteria as in the firstembodiment of the invention. It is preferable first of all to produce amixture of suboxide powder and some of the metal powder, to agglomeratethis mixture, then to admit a further part of the metal powder, followedby milling of this mixture then a further agglomeration step.

The powders are then pressed together with a niobium or tantalum wire toform anode bodies and sintered. The sintering may be carried out underhigh vacuum, producing anode structures which include both oxidic andmetallic regions.

According to a third embodiment of the invention, a suboxide powder ofcomposition NbO_(x) where 1.3<x<2.1, preferably 1.8<x<2.1, particularlypreferably 1.9<x<2, is mixed with a quantity of a metal powder which issuch that a mean composition of the mixture which corresponds to theformula NbO_(y) where 0.7<y<1.3, preferably 0.9<y<1.15, particularlypreferably 1<y<1.05, results.

The powder mixture is filled into press moulds, surrounding a contactwire made from niobium or tantalum, pressed to a green density of 2.3 to3.7 g/cm³ and sintered to form anode structures.

According to this third embodiment of the invention, however, thesintering of the anode pressed bodies to form the anode body is carriedout in a hydrogen-containing atmosphere, in such a way that oxygenexchange between the suboxide and metal particles also takes place viathe gas phase (intermediate formation of water vapour molecules at theoxide surfaces and reduction of these molecules at the metal surfaces)of the agglomerates.

In this third embodiment of the invention, it is preferable for anatmosphere with a relatively low hydrogen partial pressure to be usedduring the sintering, in order to ensure that there is no hydrogenembrittlement of the metallic component, in particular of the niobium ortantalum wire. It is preferable for the sintering to be carried outunder a gas pressure of from 10 to 50 mbar absolute. If appropriate,post-sintering can be carried out under high vacuum.

During the sintering with oxygen equalization (“reaction sintering”),the volume of the metallic starting agglomerates increases and thevolume of the oxidic starting agglomerates decreases. If a startingoxide of the approximate formula NbO₂ is used, the total volume duringoxygen equalization to form NbO remains approximately constant.Competing changes in length and volume during sintering therefore onlyoccur in the near region and are absorbed by the near region shiftswhich are in any case caused by the sintering process.

According to this third embodiment of the invention, anode bodies areformed with a substantially homogenous oxide composition of formulaNbO_(y) with y as defined above.

According to a fourth embodiment of the invention, firstly, as in thesecond embodiment of the invention, agglomerates (tertiary particles)are produced, including both metallic primary particles and/or secondaryparticles and oxidic primary and/or secondary particles within aparticles composite (tertiary agglomerate particle).

According to this fourth embodiment of the invention, a suboxide powderof composition NbO_(x) where 1 .3<x<2.1, preferably 1 .8<x<2.1,particularly preferably 1.9<x<2, is mixed with a quantity of a metalpowder which is such that a mean composition of the mixture whichcorresponds to the formula NbO_(y) where 0.7<y<1.3, preferably0.9<y<1.15, particularly preferably 1<y<1.05, results.

According to this fourth embodiment of the invention, the sintering ofthe pressed anode structures is carried out in the same way as in thethird embodiment of the invention, i.e. in the presence of hydrogen,resulting in an anode structure having a substantially homogenouscomposition corresponding to the formula NbO_(y) where 0.7<y<1.3,preferably 0.9<y<1.15, particularly preferably 1<y<1.05.

All four embodiments of the invention exploit the increased sinteringactivity of the anode pressed bodies through reaction sintering. Thisallows a considerable reduction in the sintering temperature and/or thesintering time. Both the anode pressed bodies and the sintered anodestructures have an increased compressive strength. The anchoring of thecontact wire to the anode sintered body is also improved. The anodeshave an increased wire detachment strength under tension.

Production of the suboxide powders that can be used in accordance withthe invention does not present any particular difficulty for the personskilled in the art. It is preferable to use the standard metallurgicalreaction and alloying process, according to which, as in the presentcase, a mean oxide content is produced by exposing a highly oxidizedprecursor and an unoxidized precursor, in a non-oxidizing, preferablyreducing atmosphere, to a temperature at which oxygen concentrationequalization takes place. Although processes other than this solid-statediffusion process are conceivable, they require control and monitoringfunctions which can scarcely be achieved in technical terms atacceptable levels of outlay. Therefore, according to the invention it ispreferable to use a niobium pentoxide which is commercially availablewith a high purity and to mix it with high-purity niobium metal, both inpowder form corresponding to the stoichiometry, and to treat the mixturefor several hours at a temperature of 800 to 1600° C. under an argonatmosphere which preferably contains up to 10% by volume of hydrogen. Itis preferable for both the pentoxide and the metal to have primaryparticle sizes which, after the oxygen equalization, corresponds to thedesired primary particle size of less than or slightly greater than 1 μm(smallest) cross-sectional dimension.

The niobium metal required for oxygen exchange with niobium pentoxide ispreferably produced by reducing high-purity niobium pentoxide to themetal. This can be effected aluminothermically by igniting an Nb₂O₅/Almixture and washing out the aluminum oxide formed and then purifying theniobium metal ingot by means of electron beams. The niobium metal ingotobtained after reduction and electron beam melting can be embrittledusing hydrogen in a known way and milled, producing plateletlikepowders.

The preferred process for producing the niobium metal follows thedisclosure of WO 00/67936 A1. According to this preferred 2-stageprocess, the high-purity niobium pentoxide powder is firstly reduced bymeans of hydrogen at 1000 to 1600° C., preferably at 1450° C., to formthe niobium dioxide of approximate formula NbO₂, and then the latter isreduced using magnesium vapour at 750 to 1100° C. to form the metal.Magnesium oxide which is formed in the process is washed out by means ofacids.

The preferred process for producing the niobium suboxide of formulaNbO_(x) where 1.3<x<2.1, preferably 1.8<x<2.1, particularly preferably1.9<x<2, is carried out in accordance with the first stage of theprocess disclosed in WO 00167936 A1, i.e. by reducing the niobiumpentoxide by means of hydrogen at 1000 to 1600° C.

EXAMPLES

Various powders are produced using the process described in WO 00/67936A1 from a partially agglomerated, finely particulate niobium pentoxidewhich has been screened through a screen with a mesh width of 300 μm andwhich comprises spherical primary particles with a diameter ofapproximately 0.4 μm, for the following experiments:

Powder 0: The niobium pentoxide powder is reduced to NbO₂ at 1250° C.under flowing hydrogen.

Powder A: The niobium pentoxide powder is reduced to form NbO2 at 1480°C. under flowing hydrogen, milled and rubbed through a screen with amesh width of 300 μm.

Powder B: Powder 0 is reduced to the niobium metal by means of magnesiumvapour at a temperature of 980° C., milled, agglomerated in vacuo at1150° C., cooled, passivated by gradual admission of oxygen and rubbedthrough a screen with a mesh width of 300 μm.

Powder C: Powder A and powder B are mixed in a molar ratio of 1:1,gently milled, heated to 1400° C. under an atmosphere comprising 80% byvolume of argon and 20% by volume of hydrogen and rubbed through ascreen with a mesh width of 300 μm.

Powder D: Powder A and powder B are mixed in a molar ratio of 1:0.8,heated to 1400° C. under an atmosphere comprising 80% by volume of argonand 20% by volume of hydrogen, and then rubbed through a screen with amesh width of 300 μm.

Powder E: Powder A and powder B are mixed in a molar ratio of 1:0.7,heated to 1400° C. under an atmosphere comprising 80% by volume of argonand 20% by volume of hydrogen and then rubbed through a screen with amesh width of 300 μm.

Table 1 gives the properties (mean values) for the powders obtained.

Mixtures were produced from the powders A, B, C, D and B, and thesemixtures were used to produce anodes. The conditions are given in Table2:

TABLE 1 Powder Powder Powder Powder Powder A B C D E NbO_(1.97) NbNbO_(0.98) NbO_(1.21) NbO_(1.32) Primary particle μm 0.87 0.75 0.96 1.11.1 size¹⁾ Agglomerate D10, 43 37 58 67 56 size²⁾ μm D50, 128 117 145151 164 μm D90, 254 248 272 281 293 μm BET surface m²/g 1.6 1.05 1.1 1.11.1 area³⁾ Flow properties⁴⁾ s 30 28 59 60 58 ¹⁾determined visually fromREM images. ²⁾laser diffraction (Malvern Mastersizer), ASTM B 822,wetting agent Daxad 11 ³⁾ASTM D 3663 ⁴⁾in accordance with Hall, ASTM B213, duration of flow for 25 g of powder

First of all “powder preforms” were produced from the powders byintroducing them into suitable press tools, into which a contact wiremade from tantalum had been placed, and pressing to a green density of2.8 g/cm³, and these powder preforms, standing freely in a furnace, weresintered at the temperature indicated either under a pressure of 10⁻⁵bar (vacuum) or at standard pressure in the atmosphere indicated.

To determine the compressive strength of the pressed and sinteredbodies, cylindrical pressed bodies with a green density of 2.8 g/cm³were produced with dimensions 3.6 =m diameter and 3.6 mm length with aweight of 106 mg without fitted contact wire and sintered whereappropriate.

TABLE 2 Mixing ratio of the powders Pretreatment of the Exam- (parts bypowders prior to ple weight) production of the Sintering No A:B:C:D:Epressed bodies conditions  1 0:0:100:0:0 ./. Vacuum, 1450° C. (Comp.)  20:10:90:0:0 Mixing Vacuum, 1350° C.  3 0:20:80:0:0 Mixing Vacuum, 1300°C.  4 0:30:70:0:0 Mixing Vacuum, 1270° C.  5 0:40:60:0:0 Mixing Vacuum,1240° C.  6 0:20:80:0:0 Mixing, agglomeration⁵⁾, Vacuum, 1350° C. 1250°C., argon; Milling, agglomeration, screening⁶⁾  7 0:30:70:0:0 Mixing,agglomeration, Vacuum, 1270° C. 1250° C., argon, milling, agglomeration,screening  8 57:43:0:0:0 Mixing 90 Ar + 10 H₂ 1300° C.  9 57:43:0:0:0Mixing 90 Ar + 10 H₂ 1250° C. 10 57:43:0:0:0 Mixing 90 Ar + 10 H₂ 1200°C. 11 57:43:0:0:0 Mixing, agglomeration, 90 Ar + 10 H₂ 1150° C., argon,1260° C. milling, screening 12 57:43:0:0:0 Mixing, agglomeration, 90Ar + 10 H₂ 1150° C., argon, 1260° C. milling, agglomeration, screening13 0:20:0:80:0 Mixing 95 Ar + 5 H₂, 1270° C. 14 0:30:0:0:70 Mixing 95Ar + 5 H₂, 1245° C. ⁵⁾“Agglomeration” means that the powders were heatedat the temperature indicated in the atmosphere indicated to formsintered bridges over a period of 20 minutes. ⁶⁾Rubbing through a screenwith a mesh width of 300 μm.

TABLE 3 Powder properties after pretreatment Anode/capacitor propertiesCompressive Compressive Wire detach- Flow strength of strength mentspec. prop- the of the strength capac- Ex. erties powder sintered underitance No. s preform⁷⁾ kg body⁸⁾ kg tension⁹⁾ kg μFV/g 1 59 0.5 5.2 1.577,131 (Comp.) 2 49 1.5 10.8 2.4 75,837 3 41 2.1 13.7 2.8 77,792 4 352.5 15.1 3.1 76,232 5 30 2.8 16.4 3.3 74,566 6 40 2.3 15.1 3.0 77,924 737 2.7 16.9 2.9 78,411 8 37 2.5 17.3 3.0 68,442 9 37 2.5 17.3 3.0 73,97810 37 2.5 17.3 3.0 78,112 11 41 2.4 18.9 2.7 75,336 12 40 2.8 19.1 2.973,592 13 42 2.0 12.9 2.6 78,618 14 37 2.3 14.7 2.8 79,915 ⁷⁾the pressedbody without contact wire was clamped between the jaws of acompressive-force measurement apparatus and the jaws were pressedtogether until the pressed body disintegrated. ⁸⁾as under ⁷⁾, butmeasured after sintering. ⁹⁾the anode body was clamped at the peripheryin a threaded clamp, the contact wire was connected to a tension deviceand tensile load was applied until the wire became detached undertension.

Cylindrical anode bodies with a tantalum wire inserted axially in thecentre, with a diameter of 3.6 mm and a length of 3.6 mm and an initialweight of powder of 103 mg, were produced in order to determine thecapacitor properties and the wire detachment strength under tension.

The anode structures were then formed in 0.1% by weight strengthphosphoric acid up to a forming voltage of 30 V at a current intensitylimited to 150 mA/g, with the voltage being maintained for over 2 hafter the current intensity had dropped to 0. To measure the specificcapacitance, the cathode used was an 18% by weight strength sulphuricacid solution, and the measurement was carried out at a bias voltage of10 V and an AC voltage with a frequency of 120 Hz.

Although the examples given above cannot yet be considered to have beenoptimized with regard to the process parameters selected, the advantagesare clearly apparent and are very promising, even if the specificresidual currents in some cases (with a high statistical scatter)reached 2 nA/μFV and were on average approximately 1 nA/μFV. Initialtests reveal the likelihood that the positive effects will be evengreater with more finely particulate powders, i.e. powders which aresuitable for capacitors with a higher capacitance, e.g. of over 120,000μFV/g.

Although the invention has been described in detail in the foregoing forthe purpose of illustration, it is to be understood that such detail issolely for that purpose and that variations can be made therein by thoseskilled in the art without departing from the spirit and scope of theinvention except as it may be limited by the claims.

1-14. (canceled)
 15. A powder for the production of anode structures forsolid electrolyte capacitors, comprising tertiary agglomerate particles,the tertiary particles being agglomerates of: (i) a first memberselected from the group consisting of primary particles of niobiumsuboxide, secondary particles of niobium suboxide and combinationsthereof; and (ii) a second member selected from the group consisting ofprimary particles of niobium metal, secondary particles of niobium metaland combinations thereof
 16. A powder mixture, optionally comprisingpowder agglomerates, having a mean composition of the following formula,NbO_(x) wherein 0.7<x<1.3, which after being pressed to a green densityof 2.8 g/cm³ has a compressive strength of over 2 kg.
 17. A pressed bodycomprising the powder mixture of claim 15 that has been pressed to adensity of from 2.3 to 3.7 g/ cm³.
 18. A pressed body comprising thepowder mixture of claim 16 that has been pressed to a density of from2.3 to 3.7 g/ cm³.
 19. A solid electrolyte capacitor anode comprising asponge-like sintered structure, the sintered structure having regionswhich comprise niobium metal, and having regions which comprise niobiumsuboxide represented by the following formula,NbO_(x) wherein 0.7<x<1.3.
 20. A solid electrolyte capacitor anodehaving a mean composition of formulaNbO_(x) wherein 0.7<x<1.3, and having a wire detachment strength undertension of over 2.0 kg.
 21. A solid electrolyte capacitor anode having amean composition of formula NbO_(x), wherein 0.7<x<1.3, and having acompressive strength of over 10 kg.
 22. A solid electrolyte capacitorcomprising the anode of claim
 19. 23. A solid electrolyte capacitorcomprising the anode of claim
 20. 24. A solid electrolyte capacitorcomprising the anode of claim 21.